18-20 inches.

Countertop height is usually 36" tall and toe kick is usually 4" so 32" is most standard.

Sorry, I wasn't clear. bepsf got it -- top of countertop to bottom of upper wall cabinets. 20" doesn't sound like much, but I trust you on that.

18" is standard for US installations from countertop to lower edge of wall cabinet. If you are installing for use by someone using a wheelchair, it is customary to include some at a lower height, but most countertop appliances will require close to the 18" clearance. If you use a large light rail at the bottom of the cabinets (to conceal undercabinet lighting), that will shorten the clearance height.